This invention deals with an apparatus for loading and unloading furnaces for processing materials such as semiconductor wafers. In preparing such material it is frequently necessary to isolate them in a vessel to heat them, oxidize them, effect diffusion or to have other physical or chemical processes performed. All of these processes, whether involved with heat, vacuum, pressure, or treatment with gas phase reactants require inserting the material to be treated into a vessel, sealing the vessel, effecting the treatment, and then removing the treated wafers from the vessel. Although this invention is not limited to any process, for the purposes of the description herein the vessel will be referred to as a furnace and the process will be described as a heating process.
Semiconductor material is extremely sensitive to impurities. Even very small amounts of contaminants can seriously influence the ability of semiconductor material to function properly. To avoid contamination of these materials extensive measures are taken to keep areas clean where the material is manufactured. The manufacture usually takes place in a clean room which is entered through an air lock, where air circulating into the room is filtered and otherwise cleaned of contaminants, where the materials of construction of the room and the equipment within it is carefully selected, and where even the apparel of the workers is controlled to avoid contaminating the product being made. Clean rooms are very expensive to build and maintain on a square foot basis and economy of using floor space is very important.
Even with the precautions taken in a clean room, contaminants are sometimes generated by the manufacturing process itself. Abrasion caused by sliding of parts against one another can generate particles. Usually contamination caused by abrasion can be controlled in a clean room by carefully avoiding exposing sensitive materials to such contamination. However, there are some areas where it is very difficult. One such area is the interior of a furnace in which wafers of semiconductor material are heated.
Wafers of semiconductor material are usually loaded into a furnace by first being loaded into a device called a boat. In a boat each wafer is held in its desired position out of contact with adjacent wafers. When the boat is loaded it is placed on a carrier which usually has a long paddle-like element on which one or more boats are carried. The carrier is then pushed into a furnace so that the wafers contained in the boats can be treated. Even though the boat carrier is usually made of quartz, which is not a contaminating material, abrasion caused by sliding on the furnace floor creates particles that detract from the quality of the semiconductor material. Boat carriers have been provided with skids and with wheels in an attempt to avoid producing particles by abrasion. However, even the skids cause abrasion and it is difficult to find wheels and axles that can be made from materials that withstand the conditions in the interior of this furnace or are not in themselves contaminants. In addition, even a carrier having wheels must have bearings and the bearing wear between the wheel and its axle produces particles that detract from the quality of the final product.
Another problem with making semiconductor material is avoiding crystal defects or even destruction of crystals due to thermal shock when a boat is placed in or removed from a furnace. In the past thermal shock has been avoided by creating what is known as a thermal ramp that in turn is created by pushing a boat into a furnace slowly and when processing is completed removing the boat from the furnace slowly. In order for the process to produce consistent material, it must be effected with reproduceable thermal ramps.